Cross-canyon variability in zooplankton backscattering strength in a river-influenced upwelling area

Abstract. Zooplankton are a key component of food webs in upwelling systems. Their distribution is affected not only by mesoscale and climate dynamics, but also by topography and local currents. Submarine canyons that cut the continental shelf can act as conduits that transport deep nutrient-rich waters to shallower areas, promoting coastal biological productivity. Consequently, canyons facilitate the advection and accumulation of zooplankton. We aimed to describe the spatio-temporal variability in zooplankton distribution (from net samples and acoustic data) and their association with local currents, in a long and narrow submarine canyon located in the highly productive continental shelf of central Chile. The backscattering strength (Sv), a proxy for zooplankton biomass, was highly variable at a diurnal and spatial scale. Higher Sv and abundances were found during nighttime, following the classic diel vertical migration pattern. Zooplankton was not uniformly distributed within the canyon. In the surface and mid-depth layers, the canyon walls accumulated more zooplankton than the center of it, specially during the night. Within the canyon, the currents were asymmetrical and frequently changed direction. When the positive along-canyon current was more intense in the northern than in the southern slope, Sv was also higher to the north. This pattern was clearer in the section closer to the canyon head. We show that submarine canyons are highly dynamic environments where conditions can rapidly change and currents revert. Our findings suggest a possible mechanism for zooplankton retention based on the asymmetry of canyon currents and the changes in horizontal zooplankton distribution.